1. Field of the Invention
This invention relates to the formation of an electrodeposit of chromium upon a conductive substrate. More particularly, the invention relates to the electrodeposition of chromium capable of forming a fine, uniform network of microcracks. This network provides improved corrosion protection for the substrate. The chromium is deposited from an aqueous bath containing a specific organic sulfur compound in an amount effective to produce a uniform microcrack pattern. Suitable organic sulfur compounds are thio diethanol, thio diacetaldehyde, thio diacetic acid and the corresponding sulfinyl and sulfonyl derivatives thereof. Other suitable compounds are disulfonated ethanol, disulfonated acetaldehyde, disulfonated acetic acid and the acid salts thereof.
2. Discussion of the Prior Art
In the field of decorative chromium electroplating, a principal objective has been traditionally to obtain a uniformly bright or lustrous deposit that provides adequate corrosion protection to the article being electroplated. This protection was thought to relate to and be dependent on the continuity of the electrodeposits.
Recently, however, the advantage of producing a discontinuous chromium layer having a fine system of micropores or microcracks has been recognized. These micropores or microcracks result in a substantial improvement in the corrosion protection of the plated article. Various mechanisms have been used for developing the microcracks or micropores including the following:
A. Electrodepositing onto a conductive substrate a highly stressed layer of nickel followed by a stressed layer of chromium, with the two layers interacting to develop a pattern of cracks. This is more completely described in U.S. Pat. No. 3,563,864.
B. Depositing discrete solid particles in or on a layer of nickel or cobalt followed by the electrodeposition of a thin chromium layer in a discontinuous manner around each particle. An example of this is shown in U.S. Pat. No. 3,152,971.
C. Lightly impinging small particles of sand or the like against a thin layer of chromium to form micropores extending through the layer and into the layer of nickel or cobalt therebeneath. This is more completely described in U.S. Pat. No. 3,625,039.
D. Inducing stress in the chromium layer by incorporating a suitable microcracking agent or stress producer into the chromium plating bath as described in U.S. Pat. No. 3,758,390. The crack throw and covering power of the bath is not, however, totally satisfactory.